To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, EUV lithography of wavelength 13.5 nm, and double patterning version of the ArF lithography, on which active research efforts have been made.
With respect to high-energy radiation of very short wavelength such as electron beam (EB) or x-ray, hydrocarbons and similar light elements used in ArF resist materials have little absorption. Then polyhydroxystyrene base resist materials are under consideration.
Resist materials for EB lithography are practically used in the mask image writing application. The exposure system for mask manufacturing made a transition from the laser beam exposure system to the EB exposure system to increase the accuracy of line width. Since a further size reduction becomes possible by increasing the accelerating voltage of the electron gun in the EB exposure system, the accelerating voltage increased from 10 kV to 30 kV and reached 50 kV in the current mainstream system, with a voltage of 100 kV being under investigation.
As the accelerating voltage increases, a lowering of sensitivity of resist film becomes of concern. As the accelerating voltage increases, the influence of forward scattering in a resist film becomes so reduced that the contrast of electron image writing energy is improved to ameliorate resolution and dimensional control whereas electrons can pass straightforward through the resist film so that the resist film becomes less sensitive. Since the mask exposure tool is designed for exposure by direct continuous writing, a lowering of sensitivity of resist film leads to an undesirably reduced throughput. Due to a need for higher sensitivity, chemically amplified resist compositions are contemplated.
Thinning of resist film is in progress to facilitate reduction of pattern feature in the EB lithography for mask manufacturing and to prevent the pattern from collapsing due to a higher aspect ratio during development. In the case of photolithography, a thinning of resist film greatly contributes to resolution improvement. This is because introduction of chemical mechanical polishing (CMP) or the like has driven forward device planarization. In the case of mask manufacture, substrates are flat, and the thickness of processable substrates (e.g., Cr, MoSi or SiO2) is predetermined by a percent light shield or phase shift control. The resist film must be improved in dry etch resistance before it can be reduced in thickness.
It is generally believed that there is a correlation between the carbon density and the dry etch resistance of resist film. As carbon density increases, etch resistance improves. Indene copolymers described in Patent Document 1 and acenaphthylene copolymers described in Patent Document 2 are expected to have a high carbon density and improved etch resistance due to a robust main chain of cycloolefin structure.
Also, with respect to the soft x-ray (EUV) lithography at wavelength 5-20 nm, the reduced absorption of carbon atoms was reported. Increasing the carbon density is effective not only for improving dry etch resistance, but also for increasing the transmittance in the soft x-ray wavelength region.
As the feature size is reduced, image blurs due to acid diffusion become a problem. To insure resolution for fine patterns with a size of 45 nm et seq., not only an improvement in dissolution contrast is requisite, but control of acid diffusion is also important, as known from previous reports. Since chemically amplified resist compositions are designed such that sensitivity and contrast are enhanced by acid diffusion, an attempt to minimize acid diffusion by reducing the temperature and/or time of post-exposure bake (PEB) fails, resulting in drastic reductions of sensitivity and contrast. Since the distance of acid diffusion is closely related to the type of acid labile group, it would be desirable to have an acid labile group which permits deprotection reaction to proceed at a very short distance of acid diffusion.
It is pointed out that an ArF resist material (typically methacrylic acid) having a carboxyl group substituted with an acid labile group swells in alkaline developer. On the other hand, a KrF resist material (typically hydroxystyrene) having a phenolic hydroxyl group substituted with an acid labile group little swells. However, hydroxystyrene allows for substantial acid diffusion, indicating a decline of resolution. There is a desire to have a resist material featuring reduced acid diffusion and least swell in alkaline developer.
Patent Document 3 describes hydroxyphenyl methacrylate as adhesive group. It is effective for reducing swell like hydroxystyrene and more effective for suppressing acid diffusion than hydroxystyrene. Patent Document 3 also describes a hydroxyphenyl methacrylate substituted with an acid labile group.
Addition of an acid generator capable of generating a bulky acid is effective for suppressing acid diffusion. It is then proposed to incorporate recurring units derived from an onium salt having a polymerizable unsaturated bond in a polymer as the acid generator. Patent Documents 4 to 6 disclose sulfonium salts having a polymerizable unsaturated bond, capable of generating a sulfonic acid and similar iodonium salts.